The German Prestandard DIN V VDE V 0126-1-1 requests that an inverter be separated from a public power grid within specified cut-off periods which depend on the effective value of a differential current suddenly occurring over its grid connections. As already mentioned, the differential current, besides a resistive fault current component, consists of an additional capacitive leakage current component, and these two components are vectorially added to give the differential current. From the requirements of the standardized test arrangement and test procedure for fulfilling said standards, it has to be taken that a quick increase in the fault current alone—even with a dominating high constant leakage current—has to result in a separation of the inverter from the grid. As a rule, the differential current is measured with a differential current transformer whose voltage signal is a measure of the differential current between the phases and the neutral line of the inverter. To ensure the detection of any step of the fault current component as requested by said standards, the fault current component of the differential current has to be determined. Due to the tendency towards developing inverters with increasing power and larger dimensions of the photovoltaic units but without a transformer, the capacitances with respect to earth potential and hence the occurring leakage currents are increased. Due to the use of certain materials for photovoltaic units, this tendency is even increased further. For the purpose of sufficiently sensitively measuring any step of the fault current, the leakage current has to be separated from the differential current. Further, the detection of a step of the fault current becomes increasingly difficult as the sensitivity of the differential current transformer is generally reduced by high leakage currents and thus strongly reduced for fault current occurring at the same time.
For the purpose of separating leakage currents from the voltage signal of a differential current transformer it is known to define all currents measured by the differential current transformer which only vary slowly as leakage currents and to compensate them by an evaluation software. Occurring steps of the fault current may then be calculated vectorially, and they are detectable up to the resolution limit of the digitization of the voltage signal of the differential current transformer. In this procedure, however, any steps of the leakage current will erroneously be interpreted as fault currents. Additionally, the resolution of the digitization limits the maximum leakage current at which steps of the fault current are still detected as requested.
From EP 1 229 629 A2 it is known to filter a differential current between the input lines of an inverter connected to a photovoltaic unit by means of an electric circuitry to determine the fault current component of the differential current. Specifically, frequency components of the inverter having typical frequencies of the leakage current generated by the operation of the inverter are filtered off to remove the leakage current component from the differential current. Particularly, with the limitation to only two main frequencies accomplished here, this known method proves insufficient to effectively remove the essential leakage current component from the differential current in practice.
Further, a method of and a device for determining a fault current component of a differential current which is measured as a current sum over a plurality of lines carrying a current of an AC current generator are known from EP 1 229 629 A2. By measuring the voltages between the input lines of an inverter of the AC current generator and earth potential, more precisely, by measuring the AC voltage component of these voltages, the driving force of the leakage current included in the differential current is determined. Starting from this voltage signal and fixed stray capacitances, the leakage current is calculated and subtracted from the differential current to obtain the fault current. For example with a photovoltaic unit as a DC current source of the AC current generator, the stray capacitances, however, are not constant but inter alia vary due to rainfall on the photovoltaic panels. Thus, it is not possible to determine the actual leakage current from the voltages of the input lines with respect to earth potential when using a fixed value of the stray capacitances.
A method and an installation for isolation and fault current surveillance in an electric AC current grid, in which a differential current formed by vectorial addition is measured between at least two grid lines and in which the AC current component of the differential current and the phase angle φ indicating the real power of the AC current component are determined, are known from DE 198 26 410 A1. A load is not only disconnected, if the differential current exceeds a certain operation value, but also, if the product of the amplitude of the AC current component of the differential current and the cosine of the phase angle φ exceed a predefined operation value. A practical directive how to determine the entire fault current component of the differential current in a simple way, however, is not given by this document.
There still is a need for a method of and a device for determining a fault current component of a differential current, in which the fault current component of the differential current is determined, i.e. separated from the leakage current component of the differential current independently of how high a DC current component and an AC current component of the fault current are, and independently of the extent to which the relevant stray capacitances vary.